Method and apparatus for transmitting channel state information

ABSTRACT

A method for transmitting channel status information in a wireless communication system. A user equipment (UE) transmits first channel information indicating a set of precoding matrices to a base station (BS) periodically, and transmits second channel information indicating one precoding matrix included in the set of precoding matrices to the BS periodically. The second channel information transmitted in a specific subframe is determined based on first channel information that has most recently been transmitted before the specific subframe. Additionally, the UE calculates a channel quality indicator (CQI) transmitted in the specific subframe based on one precoding matrix specified by the first channel information that has most recently been transmitted before the specific subframe and by the second channel information determined based on the first channel information that has most recently been transmitted before the specific subframe.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/806,642 filed on Dec. 21, 2012, which is the national phaseof PCT International Application No. PCT/KR2011/004492 filed on Jun. 20,2011, which claims the benefit of U.S. Provisional Application No.61/357,514 filed on Jun. 22, 2010. The entire contents of all of theabove applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication and, morespecifically, to a method and apparatus for transmitting channel statusinformation in a wireless communication system.

Discussion of the Related Art

One of the most important requirements of the next-generation wirelesscommunication system is to support a requirement for a high datatransfer rate. To this end, research is being carried out on varioustechnologies, such as Multiple Input Multiple Output (MIMO) andCooperative Multiple Point transmission (CoMP), and a relay, but themost basic and stable solution is to increase the bandwidth.

However, frequency resources are now in the saturation state, andvarious technologies are partially being used in wide-ranging frequencybands. For this reason, as a scheme for securing a wide bandwidth inorder to satisfy requirements for a higher data transfer rate, a CarrierAggregation (CA) having a concept that each of scattered bands isdesigned to satisfy a basic requirement capable of operating anindependent system and a number of bands are bundled into one system isbeing introduced. Here, a band that can be independently operated isdefined as a Component Carrier (CC).

In order to support an increasing transmission capacity, in a recentcommunication standard, for example, a standard, such as 3GPP LTE-A or802.16m, the extension of a 20 MHz or higher bandwidth is taken intoconsideration. In this case, a broadband is supported by aggregating oneor more CCs. For example, if one CC corresponds to a bandwidth of 5 MHz,a bandwidth of a maximum of 20 MHz is supported by aggregating fourcarriers. A system using a CA as described above is called amulti-carrier system.

Meanwhile, for the purpose of efficient communication between a basestation and a terminal, channel status information needs to be fed back.Channel state information fed back from a terminal and a base stationcan be plural according to circumstances, and one piece of channelstatus information can be generated and interpreted based on the otherpiece of channel status information. In this case, for example, the onepiece of channel status information cannot be transmitted for somereasons and only the other piece of channel status information can betransmitted. Here, whether a terminal will transmit the other piece ofchannel status information or not using what method can be problematic.Furthermore, from a viewpoint of a base station, how the other piece ofchannel status information will be interpreted and applied can beproblematic.

SUMMARY OF THE INVENTION

There are provided a method and apparatus in which UE transmits channelstatus information in a wireless communication system.

A method of a mobile station sending channel status information in amulti-carrier system in accordance with an aspect of the presentinvention includes receiving a reference signal from a base station;transmitting first channel information to the base station; andtransmitting second channel information to the base station, wherein thefirst channel information and the second channel information are piecesof information combined to indicate one precoding matrix estimated usingthe reference signal.

The first channel information may include information indicative of oneor more precoding matrices estimated using the reference signal, and thesecond channel information may include information indicative of any oneof the one or more precoding matrices indicated by the first channelinformation.

The first channel information may be configured so that the firstchannel information is transmitted in subframes having a first period,the second channel information may be configured so that the secondchannel information is transmitted in subframes having a second period,and the first period may be greater than the second period.

If the first channel information is dropped in a specific subframeincluded in the subframes having the first period, the second channelinformation transmitted after the specific subframe may be determinedbased on the first channel information that has most recently beentransmitted based on the specific subframe.

A Channel Quality Indicator (CQI) transmitted after the specificsubframe may be generated based on a precoding matrix specified by thefirst channel information that has most recently been transmitted basedon the specific subframe and the second channel information determinedbased on the first channel information that has most recently beentransmitted.

The first channel information and the second channel information may begenerated for each of a plurality of downlink component carriers andtransmitted.

If the first channel information on any one of the plurality of downlinkcomponent carriers is dropped in a specific subframe, the second channelinformation after the specific subframe may be determined based on thefirst channel information on the any one downlink component carrier thathas most recently been transmitted based on the specific subframe.

The first channel information and the second channel information may begenerated for each of a plurality of downlink component carrier groupsand transmitted.

If the first channel information on any one of the plurality of downlinkcomponent carrier groups is dropped in a specific subframe, the secondchannel information after the specific subframe may be determined basedon the first channel information on the any one downlink componentcarrier group that has most recently been transmitted based on thespecific subframe.

The first channel information and the second channel information may betransmitted through a physical uplink control channel.

A mobile station in accordance with another aspect of the presentinvention includes a Radio Frequency (RF) unit transmitting andreceiving radio signals and a processor connected to the RF unit,wherein the processor receives a reference signal from a base stationand transmits first channel information and second channel informationto the base station, and the first channel information and the secondchannel information are pieces of information combined to indicate oneprecoding matrix estimated using the reference signal.

The first channel information may include information indicative of oneor more precoding matrices estimated using the reference signal, and thesecond channel information may include information indicative of any oneof the one or more precoding matrices indicated by the first channelinformation.

The first channel information may be configured so that the firstchannel information is transmitted in subframes having a first period,the second channel information may be configured so that the secondchannel information is transmitted in subframes having a second period,and the first period may be greater than the second period.

If the first channel information is dropped in a specific subframeincluded in the subframes having the first period, the second channelinformation transmitted after the specific subframe may be determinedbased on the first channel information that has most recently beentransmitted based on the specific subframe.

Although a mobile station does not transmit some of pieces of channelstatus information in a wireless communication system, a base stationcan recognize a channel state with the mobile station. Accordingly, thereliability of communication can be improved because precoding matricesinto which a channel state between a base station and a mobile stationhas been incorporated are efficiently applied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows the structure of a radio frame.

FIG. 3 shows an example of a resource grid for one downlink slot.

FIG. 4 shows an example of the structure of a downlink subframe in 3GPPLTE.

FIG. 5 shows the structure of an uplink subframe.

FIG. 6 is a conceptual diagram showing the generation and transmissionof CQI.

FIG. 7 shows schemes for selecting a CQI subband and generating CQI inthe frequency domain.

FIG. 8, including (a) and (b), shows an example of a comparison betweenthe existing single carrier system and a multi-carrier system.

FIG. 9 illustrates the structure of a subframe for cross-carrierscheduling in a multi-carrier system.

FIG. 10 shows a method of transmitting CSI in accordance with anembodiment of the present invention.

FIG. 11 illustrates a method 1.

FIG. 12 illustrates a method 2.

FIG. 13 illustrates a method 3.

FIG. 14 shows the construction of a mobile station in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Long Term Evolution (LTE) according to the 3rd Generation PartnershipProject (3GPP) standardization organization is part of Evolved-UMTS(E-UMTS) that uses an Evolved-Universal Terrestrial Radio Access Network(E-UTRAN), and it adopts Orthogonal Frequency Division Multiple Access(OFDMA) in downlink and adopts Single Carrier-Frequency DivisionMultiple Access (SC-FDMA) in uplink. LTE-Advanced (A) is the evolutionof LTE. In order to clarify a description hereinafter, 3GPP LTE/LTE-Aare basically described, but the technical spirit of the presentinvention is not limited thereto.

FIG. 1 shows a wireless communication system.

Referring to FIG. 1, the wireless communication system 10 includes oneor more Base Stations (BSs) 11. The BSs 11 provide communicationservices to specific geographical areas 15 commonly called cells. Eachof the cells may be divided into a plurality of areas, and each of theareas is called a sector. One BS may include one or more cells. Ingeneral, the BS 11 refers to a fixed station that communicates with UEs13, and it may also be called another terminology, such as an evolvedNodeB (eNB), a Base Transceiver System (BTS), an access point, or anAccess Network (AN).

The User Equipment (UE) 12 may be fixed or mobile and may also be calledanother terminology, such as a Mobile Station (MS), a User Terminal(UT), a Subscriber Station (SS), a wireless device, a Personal DigitalAssistant (PDA), a wireless modem, a handheld device, or an AccessTerminal (AT).

Hereinafter, downlink (DL) refers to communication from the BS 11 to theUE 12, and uplink (UL) refers to communication from the UE 12 to the BS11.

The wireless communication system 10 may be a system which supportsbidirectional communication. Bidirectional communication can beperformed using Time Division Duplex (TDD) mode, Frequency DivisionDuplex (FDD) mode or the like. TDD mode uses different time resources inUL transmission and DL transmission. FDD mode uses different frequencyresources in UL transmission and DL transmission. The BS 11 and the UE12 communicate with each other using radio resources called radioframes.

FIG. 2 shows the structure of a radio frame.

Referring to FIG. 2, the radio frame includes 10 subframes, and onesubframe includes two slots. The length of one subframe may be 1 ms, andthe length of one slot may be 0.5 ms. The time that it takes to transmitone subframe is called a Transmission Time Interval (TTI). The TTI maybe a minimum scheduling unit.

One slot may include a plurality of Orthogonal Frequency DivisionMultiplexing (OFDM) symbols in the time domain. The OFDM symbol is usedto represent one symbol period because 3GPP LTE uses OFDMA in downlinkand may be called another terminology according to a multiple accessscheme. For example, if SC-FDMA is used as an uplink multiple accessscheme, corresponding symbols may be called SC-FDMA symbols. One slot isillustrated as including 7 OFDM symbols, but the number of OFDM symbolsincluded in one slot may be changed depending on the length of a CyclicPrefix (CP). In accordance with 3GPP TS 36.211 V8.5.0 (2008-12), 1subframe includes 7 OFDM symbols in a normal CP, and 1 subframe includes6 OFDM symbols in an extended CP. The structure of the radio frame isonly an example, and the number of subframes included in the radio frameand the number of slots included in the subframe may be changed invarious ways.

FIG. 3 shows an example of a resource grid for one downlink slot.

Referring to FIG. 3, the downlink slot includes a plurality of OFDMsymbols in the time domain and includes N_(RB) Resource Blocks (RBs) inthe frequency domain. The resource block is a resource allocation unit,and it includes one slot in the time domain and includes a plurality ofcontiguous subcarriers in the frequency domain.

The number of resource blocks N_(RB) included in a downlink slot dependson a downlink transmission bandwidth configured in a cell. For example,in an LTE system, the number of resource blocks N_(RB) may be any one of6 to 110. An uplink slot may have the same structure as the downlinkslot.

Each of elements on the resource grid is called a Resource Element (RE).The resource elements on the resource grid may be identified by an indexpair (k, l) within a slot. Here, k (k=0, . . . , N_(RB)×12-1) indicatesa subcarrier index in the frequency domain, and l (l=0, . . . , 6)indicates an OFDM symbol index in the time domain.

In FIG. 3, one resource block is illustrated as including 7×12 resourceelements, including 7 OFDM symbols in the time domain and 12 subcarriersin the frequency domain. However, the number of OFDM symbols and thenumber of subcarriers within a resource block are not limited thereto.The number of OFDM symbols and the number of subcarriers may be changedin various ways depending on the length of a CP, frequency spacing, etc.For example, the number of OFDM symbols is 7 in case of a normal CP, andthe number of OFDM symbols is 6 in case of an extended CP. One of 128,256, 512, 1024, 1536, and 2048 may be selected and used as the number ofsubcarriers in one OFDM symbol.

FIG. 4 shows an example of the structure of a downlink subframe in 3GPPLTE. The subframe includes two consecutive slots. A maximum of threeformer OFDM symbols of a first slot within the downlink subframe becomea control region to which a physical downlink control channel (PDCCH) isallocated, and the remaining OFDM symbols become a data region to whichphysical downlink shared channels (PDSCHs) are allocated. Controlchannels, such as a physical control format indicator channel (PCFICH)and a physical hybrid ARQ indicator channel (PHICH), in addition to thePDCCH can be allocated to the control region. UE can read datainformation transmitted through the PDSCHs by decoding controlinformation transmitted through the PDCCH. Here, the control region isillustrated as including the 3 OFDM symbols, but this is onlyillustrative. The PDCCH carries a downlink grant that informs theallocation of the resources of downlink transmission on the PDSCH. Moreparticularly, the PDCCH can carry the allocation of the resources of thetransport format of a downlink shared channel (DL-SCH), paginginformation on a paging channel (PCH), system information on a DL-SCH,the allocation of the resources of a higher layer control message, suchas a random access response transmitted on a PDSCH, a transmission powercontrol command, and the activation of a voice over IP (VoIP).Furthermore, the PDCCH carries an uplink grant that informs UE of theallocation of resources of uplink transmission. The number of OFDMsymbols included in the control region within the subframe can be knownby a PCFICH. The PHICH carries Hybrid Automatic Repeat reQuest (HARQ)acknowledgment (ACK)/negative-acknowledgement (NACK) signals in responseto uplink transmission.

FIG. 5 shows the structure of an uplink subframe.

Referring to FIG. 5, the uplink subframe can be divided into a controlregion and a data region in the frequency domain. A physical uplinkcontrol channel (PUCCH) on which uplink control information istransmitted is allocated to the control region. A physical uplink sharedchannel (PUSCH) on which data (control information may also betransmitted according to circumstances) is transmitted is allocated tothe data region. UE may transmit a PUCCH and a PUSCH at the same time ormay transmit only one of a PUCCH and a PUSCH depending on aconfiguration.

A PUCCH for an MS is allocated in the form of a resource block pair (RBpair) in the subframe. Resource blocks that belong to the resource blockpair occupy different subcarriers in a first slot and a second slot. Afrequency that is occupied by the resource blocks belonging to theresource block pair to which a PUCCH is allocated is changed on thebasis of a slot boundary. This is said that the RB pair allocated to thePUCCH has been subjected to frequency-hopped at the slot boundary. UEcan obtain a frequency diversity gain by transmitting uplink controlinformation through different subcarriers according to the time.

A Hybrid Automatic Repeat reQuest (HARQ) acknowledgement(ACK)/non-acknowledgement (NACK), and Channel Status Information (CSI)(e.g., a Channel Quality Indicator (CQI), a Precoding Matrix Index(PMI), a Precoding Type Indicator (PTI), and a Rank Indication (RI))indicating a downlink channel state can be transmitted on the PUCCH.

The PUSCH is mapped to an UL-Uplink Shared Channel (SCH), that is, atransport channel. Uplink data transmitted on the PUSCH may be atransport block, that is, a data block for the UL-SCH transmitted duringa TTI. The transport block may include user data. Or, the uplink datamay be multiplexed data. The multiplexed data may be the multiplexing ofthe transport block for the UL-SCH and channel status information. Forexample, the channel status information multiplexed into the data may bea CQI, a PMI, or an RI. Or, the uplink data may include only the channelstatus information.

1. CSI in a Wireless Communication System

A) Channel State Information (CSI)

For the purpose of efficient communication, channel information needs tobe fed back. In general, downlink channel information is transmitted inuplink, and uplink channel information is transmitted in downlink.Channel information indicating the state of a channel is called CSI, andthe CSI includes a CQI, a PMI, an RI, etc. The CQI provides informationon a link adaptive parameter that can be supported by UE for a giventime. The PMI provides information on a precoding matrix in theprecoding based on a codebook. The PMI is related to Multiple InputMultiple Output (MIMO). In MIMO, the feedback of a PMI is calledclosed-loop MIMO. Downlink transmission mode is classified into 9 typesbelow. PMI feedback is used in 4, 5, 6, and 9 in the 9 types of downlinktransmission mode. In downlink transmission mode 8, when a PMI/RI reportis set, UE feeds a PMI back.

Single antenna port: mode in which precoding is not performed.

Transmission diversity: Transmission diversity can be used in 2 or 4antenna ports which use SFBC.

Open-loop space multiplexing: open-loop mode rank adaptation based on RIfeedback can be applied. If the rank is 1, transmission diversity can beapplied. If the rank is greater than 1, great delay CDD can be used.

Closed-loop space multiplexing: mode n which precoding feedbacksupporting dynamic rank adaptation is applied.

Multi-User MIMO

Closed-loop space multiplexing having a single transmission layer

Single antenna port: mode that can be used in beamforming when aUE-specific reference signal is used. If the number of PBCH antennaports is 1, a single antenna port (port 0) is used. If not, transmissiondiversity is used.

Transmission of a dual layer: the transmission of a dual layer usingantenna ports 7 and 8 or the transmission of a single antenna port usingan antenna port 7 or an antenna port 8.

Closed-Loop Space Multiplexing.

Transmission of a maximum of 8 layers: The transmission of a maximum of8 layers using an antenna port 7 to 14. Closed-loop space multiplexing.

An RI is information on the number of layers that are recommended by UE.That is, the RI indicates the number of streams used in spacemultiplexing. The RI is fed back only when UE operates in MIMO modeusing space multiplexing. That is, the RI is fed back only in 3, 4, 8,and 9 from the 9 types of downlink transmission mode. For example, insingle antenna port mode or transmission diversity mode, the RI is notfed back. The RI may have a value 1 or 2 in a 2×2 antenna configurationand may have one of 1 to 4 in a 4×4 antenna configuration. The RI isalways related to one or more CQI feedbacks. That is, the feedback CQIis calculated under the assumption of a specific RI value. In general,the rank of a channel is changed slower than a CQI, and thus the RI isfed back by a smaller number of times than CQI. The transmission periodof the RI may be a multiple of that of a CQI/PMI. The RI is given forthe entire system band, and frequency selective RI feedback is notsupported.

The CQI can be generated in various ways. For example, there are amethod of quantizing a channel state without change and feeding thequantized channel state back, a method of calculating a Signal toInterference plus Noise Ratio (SINR) and feeding the calculated SINRback, and a method of informing a state in which the CQI is actuallyapplied to a channel, such as a Modulation Coding Scheme (MCS).

If the CQI is generated based on the MCS, the MCS includes a modulationmethod, a coding method, and a corresponding coding rate. Accordingly,if the modulation method and the coding method are changed, the CQI hasto be changed. Thus, at least one CQI per codeword is necessary.

If Multi Input Multi Output (MIMO) is applied to a wirelesscommunication system, the number of necessary CQIs is changed. That is,an MIMO system can use a plurality of codewords because it generatesmultiple channels using multiple antennas. Accordingly, the number ofcorresponding CQIs has to be plural. If a plurality of CQIs is used, theamount of corresponding control information is increased proportionally.

FIG. 6 is a conceptual diagram showing the generation and transmissionof CQI.

Referring to FIG. 6, UE measures a downlink channel state and reports aselected CQI value to a BS through an uplink control channel based onthe measured downlink channel state. The BS performs downlink scheduling(UE selection, resource allocation, etc) according to the reported CQI.Here, the CQI value may be the SINR, Carrier to Interference and NoiseRatio (CINR), Bit Error Rate (BER), or Frame Error Rate (FER) value ofthe channel or a value converted from the SINR, the CINR, the BER, orthe FER so that it can be transmitted. In the case of an MIMO system,the PMI, the RI, etc. in addition to the CQI can be added as CSI intowhich the channel state has been incorporated.

B) Characteristics in the Frequency Band of a CQI

In order to utilize a given channel capacity to a maximum in a wirelesscommunication system, an MCS and transmission power are controlledaccording to a given channel through link adaptation. In order for a BSto perform this link adaptation, UE has to feed CSI back.

If a frequency band used by a wireless communication system has abandwidth exceeding a coherence bandwidth, a channel suddenly changeswithin the bandwidth. In particular, if OFDM is used, severalsubcarriers are included in a given bandwidth and a modulated symbol istransmitted through each subcarrier. Thus, for optimal channeltransmission, a channel state has to be incorporated into eachsubcarrier. To this end, several methods for reducing overhead due to asudden increase in the amount of CSI feedback in a wirelesscommunication system in which the number of subcarriers is plural havebeen proposed.

C) Scheme for Generating CQI

A method proposed in order to reduce overhead due to an increase in theamount of transmitted CSI (e.g., CQI) is described in brief.

First, there is a method of changing a unit of CSI transmitted. Forexample, there is a method of grouping several subcarriers into onesubcarrier group and transmitting CSI, transmitted every subcarrier inan OFDM method, for each subcarrier group. For example, if 12subcarriers are grouped into one subcarrier group in an OFDM methodusing 2048 subcarriers, a total of 171 subcarrier groups are formed. Theamount of actually transmitted CSI is reduced from 2048 to 171.

If a frequency band is classified into an integer number of subcarriersas in an OFDM method, one subcarrier or a plurality of subcarriers aregrouped into one subcarrier group, and the basic unit of a method ofreporting each CQI for each subcarrier group is defined as a CQIsubcarrier group or a CQI subband. Meanwhile, if a frequency band is notclassified into each of subcarriers, the entire frequency band isclassified into some frequency bands, CQI is generated based on theclassified frequency band, and the frequency band classified in order togenerate the CQI is defined as a CQI subband.

Second, there is a method of compressing CSI. For example, there is amethod of compressing a CQI every subcarrier in an OFDM method andtransmitting the compressed CQI. Methods, such as a Discrete CosineTransform (DST), may be taken into consideration as the compressionmethod.

Third, there is a method of selecting a frequency band and generatingCSI. For example, in an OFDM method, there is a best-M method ofselecting the best M (M is a natural number) subcarriers fromsubcarriers or a subcarrier group without transmitting channelinformation in each of all the subcarriers and transmitting channelinformation through the selected M subcarriers. When selecting afrequency band and transmitting a corresponding CQI, data that isactually transmitted can be basically divided into two parts. The firstpart is a CQI value part, and the second part is a CQI index part.

D) Scheme for Generating a Frequency Band Selective CQI

FIG. 7 shows schemes for selecting a CQI subband and generating CQI inthe frequency domain.

Referring to FIG. 7, the scheme for generating a frequency bandselective CQI basically includes three parts. The first part is a methodof selecting a frequency band in which a CQI will be generated, that is,a CQI subband. The second part is a method of manipulating andgenerating the CQI values of the selected frequency bands andtransmitting the generated CQI values. The third part is a method oftransmitting the selected frequency bands, that is, the indices of CQIsubbands.

First, the method of selecting a CQI subband includes, for example, thebest-M method and a threshold-based method. The best-M method is amethod of selecting M CQI subbands having a good channel state. If thevalue of M is 3, three CQI subbands having index Nos. 5, 6, and 9 havinga good channel state are selected. The threshold-based method is amethod of selecting a CQI subband having a better channel state than apredetermined threshold. In this method, CQI subbands having index Nos.5 and 6 higher than a threshold are selected in FIG. 7.

Second, the method of generating and transmitting CQI values includes,for example, an individual transmission method and an averagetransmission method. The individual transmission method is a method oftransmitting all the CQI values of selected CQI subbands. Accordingly,in the individual transmission method, if the number of selected CQIsubbands increases, the number of CQI values to be transmitted isincreased. The average transmission method is a method of transmittingthe mean of the CQI values of selected CQI subbands. Accordingly, theaverage transmission method is advantageous in that CQI values to betransmitted are united into one irrespective of the number of selectedCQI subbands. In contrast, since the mean value of several CQI subbandsis transmitted, there is a disadvantage in that accuracy is low. In theaverage transmission method, a method of simply calculating the mean maybe a simple arithmetic average or may be the mean into which a channelcapacity has been incorporated.

Third, the method of transmitting the index of a CQI subband includes,for example, a bitmap index method and a combinatorial index method. Thebitmap index method is a method of allocating 1 bit to each of CQIsubbands, allocating 1 to the 1 bit value of a specific CQI subband ifthe specific CQI subband is used, and allocating 0 to the 1 bit value ofa specific CQI subband if the specific CQI subband is not used (ofcourse, 0 may be allocated to the 1 bit value of a specific CQI subbandif the specific CQI subband is used, and 1 may be allocated to the 1 bitvalue of a specific CQI subband if the specific CQI subband is not used)in order to indicate that what CQI subband is used. The bitmap indexmethod requires the number of bits equal to a total number of CQIsubbands, but can represent corresponding CQI subbands using a constantnumber of bits irrespective of how many CQI subbands are used. Thecombinatorial index method is a method of determining how many CQIsubbands will be used and mapping the case of a combination equal to thenumber of CQI subbands used in a total number of CQI subbands to eachindex. For example, if a total of N CQI subbands are included and M (Nand M are natural numbers and N is equal to or greater than M) CQIsubband indices from among the N CQI subbands are used, a total numberof possible combinations is calculated as in the following equation.

$\begin{matrix}{{{}_{}^{}{}_{}^{}} = \frac{N!}{{\left( {N - M} \right)!}{M!}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

The number of bits for representing a total number of combinations as inEquation 1 is calculated as in the following equation.

$\begin{matrix}{\left\lceil {\log_{2}\left( {{}_{}^{}{}_{}^{}} \right)} \right\rceil = \left\lceil {\log_{2}\left( \frac{N!}{{\left( {N - M} \right)!}{M!}} \right)} \right\rceil} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

The example of FIG. 7 corresponds to a method of selecting 3 CQIsubbands from a total of 11 CQI subbands. Thus, a total number ofpossible combinations is ₁₁C₃=165, and the number of bits forrepresenting the 165 is 8 bits.

E) Increase of the Number of Transmitted CQIs in Several Dimensions

The number of CQIs can be increased in a variety of dimensions,resulting in increased overhead.

First, an increase of CQIs in the space dimension is described below. InMIMO, several codewords may be transmitted through several layers. Here,several CQIs are necessary. For example, in 3GPP LTE, a maximum of twocodewords are possible in MIMO. Here, two CQIs are necessary. If one CQIconsists of 4 bits and the number of codewords is 2, the CQI consists ofa total of 8 bits. This CQI has to be transmitted by each MS which hasto feed a channel state back. As a result, the CQIs occupy a large partfrom a viewpoint of all radio resources. Accordingly, it is preferencethat the number of CQIs is reduced to a minimum in terms of a channelcapacity.

Second, an increase of CQIs in the frequency domain is described below.The above-described CQI is related to contents that correspond to onefrequency band. If a receiver (UE) selects a frequency band having thebest channel state and transmits only a CQI for the selected frequencyband and a sender (BS) provides service through the selected frequencyband, the CQI is necessary only in one frequency band. In this case, amore efficient method is necessary because the above method is suitablefor a single user environment, but is not suitable for multiple users.When only a CQI for one preference frequency band is transmitted, thereis no problem if frequency bands preferred by a plurality of users donot overlap with each other. If several users select a specificfrequency band as a preference frequency band at the same time, aproblem arises. In this case, users other than a selected user do notuse the frequency band. If each user transmits only a CQI for onepreference frequency band, users not selected by a BS are fundamentallyprecluded from an opportunity to be served. Accordingly, in order tosolve this problem and effectively obtain a multi-user diversity gain,it is necessary to transmit a CQI for several frequency bands. If a CQIfor several frequency bands is transmitted, the amount of transmittedCQI information is increased. For example, if three frequency bandshaving better channel states are sequentially selected and a CQI andfrequency band indicator for each of the frequency bands aretransmitted, the number of transmitted CQIs become three times, andadditional transmission is necessary for indicators indicating theselected frequency bands.

Third, an increase of CQIs may be generated in a dimension into whichboth a space and a frequency are taken into consideration. That is,several CQIs may be necessary in the space dimension, and several CQIsmay be necessary in the frequency domain.

Fourth, an increase of CQIs may be generated in other dimensions. Forexample, if a Code Division Multiple Access (CDMA) method is used, a CQIfor each spread symbol may have to be taken into consideration becausethere is a change in the signal intensity, the amount of interferencefor each spread symbol. Accordingly, an increase of CQIs in the symboldimension may be generated. In addition, an increase of CQIs in variousdimensions may be generated.

In order to reduce the number of transmitted CQIs increasing asdescribed above, a differential CQI (delta CQI) may be used.

F) Differential CQI

The cases where several CQIs are necessary in various dimensions havebeen described. If several CQIs are necessary as described above, adifferential CQI may be used in order to reduce the number oftransmitted CQIs. That is, one CQI, that is, a reference, is selected.Here, the reference CQI is normally transmitted, whereas only adifference between the reference CQI and other CQIs is transmitted. Thatis, a method similar to a differential modulation method in modulationand demodulation methods is used. Here, if several CQIs are indicated bya differential method, a large number of bits are allocated to thereference CQI value and a relatively small number of bits are allocatedto other CQI values so that the number of transmitted CQIs is reduced.

G) CQI Transmission Mode

Uplink channels used to transmit a CQI in a 3GPP LTE system are shown inTable 1 below.

TABLE 1 PERIODIC CQI APERIODIC CQI SCHEDULING METHOD TRANSMISSIONTRANSMISSION FREQUENCY NON-SELECTIVE PUCCH FREQUENCY SELECTIVE PUCCHPUSCH

As shown in Table 1, a CQI may be transmitted through a PUCCH in theperiod that is determined by a higher layer or may be transmittedthrough a PUSCH aperiodically according to the necessity of a scheduler.If a CQI is transmitted through a PUSCH, it is possible only in afrequency selective scheduling method.

1) Transmission of CQI/PMI/RI through a PUSCH after receiving a CQItransmission request signal (CQI request)

In this case, a control signal (CQI request) that requests to transmit aCQI is included in a PUSCH scheduling control signal (UL grant)transmitted through a PDCCH. Table 2 below illustrates mode when aCQI/PMI/RI are transmitted through a PUSCH.

TABLE 2 PMI Feedback Type No PMI Single PMI Multiple PMI PUSCH WidebandMode 1-2 CQI (wideband CQI) feedback UE Selected Mode 2-0 Mode 2-2 type(subband CQI) Higher Layer- Mode 3-0 Mode 3-1 configured (subband CQI)

Transmission mode in Table 2 may be indicated by a higher layer signalthat is transmitted by a BS, and all CQI/PMI/RI may be transmittedthrough the PUSCH of the same subframe. Mode 1-2, mode 2-0, mode 2-2,mode 3-0, and mode 3-1 in Table 2 are described below.

1-1) Mode 1-2

A precoding matrix is selected assuming that data is transmitted onlythrough a corresponding subband in relation to each subband. UEgenerates a CQI under the assumption of a precoding matrix selected inrelation to a system band or a band designated by a higher layer signal(this is called a band set S).

The UE transmits the CQI and the PMI value of each subband. Here, thesize of each subband may differ depending on the size of a system band.

1-2) Mode 2-0

UE selects M preference subbands for a system band or a band (band setS) that has been designated by a higher layer signal. The UE generatesone CQI value assuming that data has been transmitted in the selected Msubbands. The UE additionally generates one CQI (a broadband CQI) valuefor the system band or the band set S.

If there is a plurality of codewords for the selected M subbands, a CQIvalue for each codeword is defined in a differential form. Thedifferential CQI=an index corresponding to the CQI value for theselected M subbands−the broadband CQI index.

The UE transmits information on the positions of the selected Msubbands, the one CQI value for the selected M subbands, and the CQIvalue generated for the system band or the band set S. Here, the size ofthe subband and the M value may be different depending on the size ofthe system band.

1-3) Mode 2-2

UE selects the positions of M preference subbands and a single precodingmatrix for the M preference subband at the same time assuming that datais transmitted through the M preference subbands.

A CQI value for the M preference subbands is defined for each codeword.The UE additionally generates a broadband CQI value for the system bandor the band set S.

The UE transmits the information on the positions of the M preferencesubbands, the one CQI value for the selected M subbands, a singlePrecoding Matrix Index (PMI) for the M preference subbands, a broadbandprecoding matrix index, and the broadband CQI value. Here, the size ofthe subband and the M value may be different depending on the size ofthe system band.

1-4) Mode 3-0

UE generates a broadband CQI value. The UE generates a CQI value foreach subband assuming that data is transmitted through each subband.Here, although RI>1, the CQI value has only a CQI value for the firstcodeword.

1-5) Mode 3-1

A single precoding matrix for a system band or a band set S isgenerated. UE generates a CQI for a subband for each codeword under theassumption of the above-described single precoding matrix generated foreach subband. The UE can generate a broadband CQI under the assumptionof a single precoding matrix.

The CQI value of each subband is represented in a differential form.That is, ‘the subband CQI=a subband CQI index−broadband CQI index’. Thesize of the subband may be different depending on the size of a systemband.

2) Periodic Transmission of a CQI/PMI/RI Through a PUCCH

A CQI may be periodically transmitted through a PUCCH or through a PUSCHaccording to circumstances. Although a CQI is transmitted through aPUSCH, the contents of the CQI/PMI/RI are generated according to one ofmodes defined in Table 3.

TABLE 3 PMI Feedback Type No PMI Single PMI PUCCH Wideband Mode 1-0 Mode1-1 CQI (wideband CQI) Feedback UE Selected Mode 2-0 Mode 2-1 Type(subband CQI)

In Table 3, in case of mode 2-0 and mode 2-1, a corresponding BandwidthPart (BP) is a set of consecutively located subbands, and it may coverboth a system band or a band set S. The size of each subband, the sizeof the BP, and the number of BPs may be different depending on the sizeof a system band. Furthermore, CQIs are transmitted in ascending powersfor each BP in the frequency domain so that the system band or the bandset S can be covered.

There may be four transmission types as follows according to atransmission combination of a CQI/PMI/RI: type 1: mode 2-0, a subbandCQI (SB-CQI) of mode 2-1 is transmitted. type 2: a broadband CQI and aPMI (WB-CQI/PMI) are transmitted. type 3: an RI is transmitted. type 4:a broadband CQI is transmitted.

If an RI and a broadband CQI/PMI are transmitted, they are transmittedin subframes having different periods and offsets. If the RI and thebroadband CQI/PMI are configured so that they are transmitted in thesame subframe, the CQI/PMI are not transmitted.

The period of each of a broadband CQI/PMI and a subband CQI is P, and ithas the following characteristics.

The broadband CQI/PMI has the period of H*P. Here, H=J*K+1, J is thenumber of frequency bands, and K is a total cycle number of frequencybands. That is, the broadband CQI/PMI is transmitted in {0, H, 2H, . . .}. The subband CQI is transmitted in points of time J*K other than apoint of time at which the broadband CQI/PMI are transmitted.

The period of the RI is M times the period of the broadband CQI/PMI, andit has the following characteristics. The offset of each of the RI andthe broadband CQI/PMI is O. If the RI and the broadband CQI/PMI aretransmitted in the same subframe, the broadband CQI/PMI are nottransmitted.

All the above-described parameters P, H, K, and O are determined in ahigher layer and signalized.

Each mode of Table 3 is described.

2-1) Mode 1-0

If an RI is transmitted, the RI is generated for a system band or a bandset S and a type 3 report is transmitted. IF a CQI is transmitted, abroadband CQI is transmitted.

2-2) Mode 1-1

If an RI is transmitted, the RI is generated for a system band or a bandset S and a type 3 report is transmitted. IF a CQI/PMI are transmitted,a single precoding matrix is selected under the assumption of the mostrecently transmitted RI. A type 2 report consisting of a broadband CQI,the single precoding matrix, and a differential broadband CQI istransmitted.

2-3) Mode 2-0

If an RI is transmitted, the RI is generated for a system band or a bandset S and a type 3 report is transmitted. If a broadband CQI istransmitted, a broadband CQI is generated under the assumption of themost recently transmitted RI, and a type 4 report is transmitted. If aCQI for a selected subband is transmitted, UE selects the most preferredsubband for J BPs including N subbands and transmits the type 1 report.The type 1 report may require one or more subframes depending on a BP.

2-4) Mode 2-1

If an RI is transmitted, the RI is generated for a system band or a bandset S and the type 3 report is transmitted. If a broadband CQI istransmitted, a broadband CQI is generated under the assumption of themost recently transmitted RI, and the type 4 report is transmitted. If aCQI for selected subbands is transmitted, UE generates a single CQIvalue for the selected subbands within a BP under the assumption of themost recently transmitted PMI/RI in relation to N_(j) J BPs and adifference between the CQIs of codewords assuming that the most recentlytransmitted RI and a single precoding matrix have been used in theselected subbands when the RI is greater than 1 and transmits the type Ireport.

For the contents described with reference to Tables 1 to 3, referencecan be made to section 7.2 of ‘3GPP TS 36.213 V8.7.0 (2009-05)’.

A multi-carrier system is described below.

FIG. 8, including (a) and (b), shows an example of a comparison betweenthe existing single carrier system and a multi-carrier system.

Referring to FIG. 8(a), in the single carrier system, only one carrieris supported for an MS in uplink and downlink. The bandwidth of acarrier may be various, but the number of carriers allocated to an MS isone. In contrast, in the multi-carrier system, a plurality of CCs (DLCCs A to C and UL CCs A to C) can be allocated to an MS. For example, inorder to allocate a bandwidth of 60 MHz to an MS, 3 CCs each having 20MHz may be allocated to the MS.

Referring to FIG. 8(b), the multi-carrier system may be divided into acontiguous CA system in which aggregated carriers are contiguous to eachother and a non-contiguous CA system in which aggregated carriers arespaced apart from each other. When a multi-carrier system is simply saidhereinafter, it is to be understood that the multi-carrier systemincludes both a case where CCs are contiguous to each other and a casewhere CCs are not contiguous to each other.

A CC, that is, a target when aggregating one or more CCs may usebandwidths used in the existing system for the purpose of backwardcompatibility with the existing system. For example, a 3GPP LTE systemsupports bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20MHz. A 3GPP LTE-A system can configure a broadband of 20 MHz or higherusing only the bandwidths of the 3GPP LTE system. Or, a 3GPP LTE-Asystem may configure a broadband by defining new bandwidths withoutusing the bandwidths of the existing system.

The system band of a wireless communication system is classified into aplurality of carrier frequencies. Here, the carrier frequency means thecenter frequency of a cell. Hereinafter, a cell may mean downlinkfrequency resources and uplink frequency resources. Or, a cell may meana combination of downlink frequency resources and optional uplinkfrequency resources. Furthermore, in general, if a CA is not taken intoconsideration, one cell may always include uplink and downlink frequencyresources that form a pair. In order for packet data to be transmittedand received through a specific cell, an MS first has to complete aconfiguration for the specific cell. Here, the configuration means astate in which the reception of system information necessary to transmitand receive data to and from the specific cell has been completed. Forexample, the configuration may include an overall process of receivingcommon physical layer parameters necessary for thetransmission/reception of data, MAC layer parameters, or parametersnecessary for a specific operation in the RRC layer. Aconfiguration-completed cell is in a state in which the cell canimmediately transmit and receive packet data only it has only to receiveinformation about which the packet data can be transmitted.

A cell of a configuration-completed state may be in an activation ordeactivation state. Here, the activation refers to a state in which datais being transmitted or received or a state in which data is ready to betransmitted or received. An MS can monitor and receive the controlchannel (PDCCH) and data channel (PDSCH) of an activated cell in orderto check resources (they may be the frequency, the time, etc.) allocatedthereto.

Deactivation refers to a state in which traffic data cannot betransmitted or received, but measurement or the transmission/receptionof minimum information are possible. An MS can receive necessary SystemInformation (SI) in order to receive packets from a deactivated cell. Incontrast, the MS does not monitor or receive the control channel (PDCCH)and data channel (PDSCH) of a deactivated cell in order to checkresources (they may be a frequency, time, etc.) allocated thereto.

A cell may be classified into a primary cell, a secondary cell, and aserving cell.

The primary cell means a cell that operates in a primary frequency, acell in which an MS performs an initial connection establishmentprocedure or a connection re-establishment procedure with a BS, or acell that is indicated as a primary cell in a handover process. Thesecondary cell means a cell that operates in a secondary frequency. Thesecondary cell is configured once RRC establishment is set up and usedto provide additional radio resources.

The serving cell is formed of a primary cell in the case of an MS inwhich a Carrier Aggregation (CA) has not been configured or to which aCA cannot be provided. If a CA has been configured for an MS, the term‘serving cell’ is used to indicate a primary cell and one of allsecondary cells or a set of a plurality of secondary cells. That is, aprimary cell means one serving cell which provides security inputs andNAS mobility information in an RRC establishment or re-establishmentstate. At least one cell may be configured to form a set of servingcells along with a primary cell depending on the capabilities of UE. Theat least one cell is called a secondary cell. Accordingly, a set ofserving cells configured for one MS may be formed of only one primarycell or may be formed of one primary cell and at least one secondarycell.

A Primary Component Carrier (PCC) means a Component Carrier (CC)corresponding to a primary cell. A PCC is a CC through which an MS formsconnection or RRC connection with a BS at the early stage from amongsome CCs. A PCC is a special CC that is responsible for connection orRRC connection for signaling regarding a plurality of CCs and thatmanages UE context, that is, connection information related to an MS.Furthermore, a PCC is always in the activation state when it is in RRCconnected mode after forming connection or RRC connection with an MS.

A Secondary Component Carrier (SCC) means a CC corresponding to asecondary cell. That is, an SCC is a CC allocated to an MS in additionto a PCC and is a carrier extended for additional resource allocation,etc. by an MS in addition to a PCC. An SCC may be divided into theactivation or deactivation state.

A downlink CC (DL CC) corresponding to a primary cell is called adownlink Primary Component Carrier (DL PCC), and an uplink CC (UL CC)corresponding to a primary cell is called an uplink Primary ComponentCarrier (UL PCC). Furthermore, in downlink, a CC corresponding to asecondary cell is called a downlink Secondary Component Carrier (DLSCC). In uplink, a CC corresponding to a secondary cell is called anuplink Secondary Component Carrier (UL SCC).

A primary cell and a secondary cell have the following characteristics.

First, a primary cell is used to transmit a PUCCH. Second, a primarycell is always activated, whereas a secondary cell is a carrier that isactivated or deactivated according to specific conditions. Third, when aprimary cell experiences a Radio Link Failure (hereinafter referred toas an RLF), RRC re-establishment is triggered, or a secondary cellexperiences an RLF, RRC re-establishment is not triggered. Fourth, aprimary cell may be changed by a change of a security key or by ahandover procedure that is accompanied by a random access channel (RACH)procedure. Fifth, Non-Access Stratum (NAS) information is receivedthrough a primary cell. Sixth, a primary cell is always formed of a pairof a DL PCC and an UL PCC. Seventh, a different CC may be configured asa primary cell in each MS. Eighth, procedures, such as thereconfiguration, addition, and removal of a primary cell, can beperformed by the RRC layer. In adding a new secondary cell, RRCsignaling may be used to transmit system information about a dedicatedsecondary cell.

A DL CC may form one serving cell, or a DL CC and an UL CC may form oneserving cell through connection establishment. However, a serving cellis not formed of only one UL CC. The activation/deactivation of a CC hasthe same concept as the activation/deactivation of a serving cell. Forexample, assuming that a serving cell1 is formed of a DL CC1, theactivation of the serving cell1 means the activation of the DL CC1.Assuming that a serving cell2 is configured through connectionestablishment of a DL CC2 and an UL CC2, the activation of the servingcell2 means the activation of the DL CC2 and the UL CC2. In this sense,each CC may correspond to a cell.

The number of CCs that are aggregated between downlink and uplink may bedifferently set. A case where the number of aggregated DL CCs is thesame as the number of aggregated UL CCs is called a symmetricaggregation, and a case where the number of aggregated DL CCs isdifferent from the number of aggregated UL CCs is called an asymmetricaggregation. Furthermore, the CCs may have different sizes (i.e.,bandwidths). For example, assuming that 5 CCs are used to form a 70 MHzband, the 70 MHz band may be configured like 5 MHz CC (carrier #0)+20MHz CC (carrier #1)+20 MHz CC (carrier #2)+20 MHz CC (carrier #3)+5 MHzCC (carrier #4).

As described above, unlike a single carrier system, a multi-carriersystem can support a plurality of Component Carriers (CCs). That is, oneMS can receive a plurality of PDSCHs through a plurality of DL CCs.

A multi-carrier system can support cross-carrier scheduling.Cross-carrier scheduling is a scheduling method capable of performingthe resource allocation of a PDSCH transmitted through other CCs and/orthe resource allocation of a PUSCH transmitted through CCs other thanCCs that is basically linked to a specific CC, through a PDCCHtransmitted through the specific CC. That is, a PDCCH and a PDSCH may betransmitted through different DL CCs, and a PUSCH can be transmittedthrough UL CCs other than an UL CC that is linked to a DL CC on which aPDCCH including an UL grant has been transmitted. As described above, asystem which supports cross-carrier scheduling requires a carrierindicator for informing that a PDSCH/PUSCH that a PDCCH provides controlinformation are transmitted through what DL CC/UL CC. A field includingthis carrier indicator is hereinafter called a Carrier Indicator Field(CIF).

A multi-carrier system which supports cross-carrier scheduling mayinclude a CIF in a conventional Downlink Control Information (DCI)format. In a system which supports cross-carrier scheduling, forexample, LTE-A system, 1 to 3 bits can be extended because a CIF isadded to the existing DCI format (i.e., a DCI format used in LTE). ThePDCCH structure may reuse the existing coding method and resourceallocation method (i.e., resource mapping based on a CCE).

FIG. 9 illustrates the structure of a subframe for cross-carrierscheduling in a multi-carrier system.

Referring to FIG. 9, a BS may configure a PDCCH monitoring DL CC set.The PDCCH monitoring DL CC set includes some of all aggregated DL CCs.When cross-carrier scheduling is configured, an MS performs PDCCHmonitoring/decoding on only DL CCs that are included in a PDCCHmonitoring DL CC set. In other words, a BS transmits a PDCCH for aPDSCH/PUSCH to be scheduled through DL CCs that are included in a PDCCHmonitoring DL CC set. A PDCCH monitoring DL CC set may be configured ina UE-specific, UE group-specific, or cell-specific way.

FIG. 9 shows an example in which 3 DL CCs DL CC A, DL CC B, and DL CC C)are aggregated and the DL CC A has been set as a PDCCH monitoring DL CC.An MS can receive DL grants for the PDSCHs of the DL CC A, the DL CC B,and the DL CC C through the PDCCH of the DL CC A. DCI that istransmitted through the PDCCH of the DL CC A includes a CIF, and thus itcan indicate that the DCI is DCI for what DL CC.

A method of transmitting CSI in a multi-carrier system is describedbelow.

FIG. 10 shows a method of transmitting CSI in accordance with anembodiment of the present invention.

Referring to FIG. 10, a BS transmits configuration information to UE(S100). The configuration information includes scheduling information onChannel Status Information (CSI) that is fed back from the UE to the BS.For example, the configuration information may include a configurationindex for feedbacks, such as a CQI, a PMI, and an RI. The UE can knowthe transmission period of the CSI and subframe offset informationthrough the configuration index.

The BS transmits a reference signal to the UE (S101). For example, theBS may transmit a Channel Status Information-Reference Signal (CSI-RS)to the UE using a maximum of 8 antenna ports. That is, the downlinktransmission mode of the BS may be the above-described transmission mode9.

The UE receives the reference signal and estimates a channel with the BSbased on the received reference signal (S102). The UE transmits firstchannel information to the BS (S103) and transmits second channelinformation (S104). Here, the first channel information and the secondchannel information are pieces of CSI, and the first channel informationand the second channel information may be combined in order to indicatea channel state. For example, the second channel information isinformation that particularly specifies the first channel information,and one PMI may be indicated using both the first channel informationand the second channel information.

An application example is described below. The first channel informationmay include CSI on a band wider than that of the second channelinformation. That is, the first channel information may include a PMIfor the entire system band, and the second channel information mayinclude a PMI for the subbands of the system band.

Furthermore, the first channel information may be transmitted in a firstperiod, and the second channel information may be transmitted in asecond period. The first period and the second period may be identicalwith each other, or the first period may be greater than the secondperiod. That is, the first channel information and the second channelinformation may be transmitted at the same time, and the second channelinformation may be transmitted more frequently than the first channelinformation.

A detailed example in which one PMI is obtained by combining the secondchannel information and the first channel information is describedbelow. For example, for the purpose of a PMI feedback, codebooks, suchas Tables 5 to 12 below, may be used. In Tables 5 to 12 below, φ_(n) andv_(m) are given as in Table 4 below.

TABLE 4 φ_(n) = e^(jπn/2) v_(m) = [1 e^(j2πm/32) e^(j4πm/32)e^(j6πm/32)]^(T)

A codebook that is used by UE in order to report 1-layer CSI using theantenna ports 15 to 22 is given as in Table 5 below.

TABLE 5 i₂ i₁ 0 1 2 3 4 0-15 W_(2i) ₁ _(,0) ⁽¹⁾ W_(2i) ₁ _(,1) ⁽¹⁾W_(2i) ₁ _(,2) ⁽¹⁾ W_(2i) ₁ _(,3) ⁽¹⁾ W_(2i) ₁ _(+1,0) ⁽¹⁾ i₂ i₁ 5 6 7 89 0-15 W_(2i) ₁ _(+1,1) ⁽¹⁾ W_(2i) ₁ _(+1,2) ⁽¹⁾ W_(2i) ₁ _(+1,3) ⁽¹⁾W_(2i) ₁ _(+2,0) ⁽¹⁾ W_(2i) ₁ _(+2,1) ⁽¹⁾ i₂ i₁ 10 11 12 13 14 0-15W_(2i) ₁ _(+2,2) ⁽¹⁾ W_(2i) ₁ _(+2,3) ⁽¹⁾ W_(2i) ₁ _(+3,0) ⁽¹⁾ W_(2i) ₁_(+3,1) ⁽¹⁾ W_(2i) ₁ _(+3,2) ⁽¹⁾ i₂ i₁ 15 0-15 W_(2i) ₁ _(+3,3) ⁽¹⁾${{where}\mspace{14mu} W_{m,n}^{(1)}} = {\frac{1}{\sqrt{8}}\begin{bmatrix}v_{m} \\{\phi_{n}v_{m}}\end{bmatrix}}$

In Table 5, i₁ may be the first channel information (i.e., a first PMI),and i₂ may be the second channel information (i.e., a second PMI). Thatis, the BS cannot specify a detailed PMI using only the first channelinformation transmitted by the UE, but can know a detailed PMI throughthe second channel information. In other words, one PMI can be specifiedusing a method of indicating one or more precoding matrices through thefirst channel information and indicating any one of the one or moreprecoding matrices, indicated by the first channel information, throughthe second channel information.

A codebook that is used by UE in order to report 2-layer CSI using theantenna ports 15 to 22 is given as in Table 6 below.

TABLE 6 i₂ i₁ 0 1 2 3 0-15 W_(2i) ₁ _(,2i) ₁ _(,0) ⁽²⁾ W_(2i) ₁ _(,2i) ₁_(,1) ⁽²⁾ W_(2i) ₁ _(+1,2i) ₁ _(+1,0) ⁽²⁾ W_(2i) ₁ _(+1,2i) ₁ _(+1,1)⁽²⁾ i₂ i₁ 4 5 6 7 0-15 W_(2i) ₁ _(+2,2i) ₁ _(+2,0) ⁽²⁾ W_(2i) ₁ _(+2,2i)₁ _(+2,1) ⁽²⁾ W_(2i) ₁ _(+3,2i) ₁ _(+3,0) ⁽²⁾ W_(2i) ₁ _(+3,2i) ₁_(+3,1) ⁽²⁾ i₂ i₁ 8 9 10 11 0-15 W_(2i) ₁ _(,2i) ₁ _(+1,0) ⁽²⁾ W_(2i) ₁_(,2i) ₁ _(+1,1) ⁽²⁾ W_(2i) ₁ _(+1,2i) ₁ _(+2,0) ⁽²⁾ W_(2i) ₁ _(+1,2i) ₁_(+2,1) ⁽²⁾ i₂ i₁ 12 13 14 15 0-15 W_(2i) ₁ _(,2i) ₁ _(+3,0) ⁽²⁾ W_(2i)₁ _(,2i) ₁ _(+3,1) ⁽²⁾ W_(2i) ₁ _(+1,2i) ₁ _(+3,0) ⁽²⁾ W_(2i) ₁ _(+1,2i)₁ _(+3,1) ⁽²⁾${{where}\mspace{14mu} W_{m,m^{\prime},n}^{(2)}} = {\frac{1}{4}\begin{bmatrix}v_{m} & v_{m^{\prime}} \\{\phi_{n}v_{m}} & {{- \phi_{n}}v_{m^{\prime}}}\end{bmatrix}}$

A codebook that is used by UE in order to report 3-layer CSI using theantenna ports 15 to 22 is given as in Table 7 below.

TABLE 7 i₂ i₁ 0 1 2 0-3 W_(8i) ₁ _(,8i) ₁ _(,8i) ₁ ₊₈ ⁽³⁾ W_(8i) ₁_(+8,8i) ₁ _(,8i) ₁ ₊₈ ⁽³⁾ {tilde over (W)}_(8i) ₁ _(,8i) ₁ _(+8,8i) ₁₊₈ ⁽³⁾ i₂ i₁ 3 4 5 0-3 {tilde over (W)}_(8i) ₁ _(+8,8i) ₁ _(,8i) ₁ ⁽³⁾W_(8i) ₁ _(+2,8i) ₁ _(+2,8i) ₁ ₊₁₀ ⁽³⁾ W_(8i) ₁ _(+10,8i) ₁ _(+2,8i) ₁₊₁₀ ⁽³⁾ i₂ i₁ 6 7 8 0-3 {tilde over (W)}_(8i) ₁ _(+2,8i) ₁ _(+10,8i) ₁₊₁₀ ⁽³⁾ {tilde over (W)}_(8i) ₁ _(+10,8i) ₁ _(+2,8i) ₁ ₊₂ ⁽³⁾ W_(8i) ₁_(+4,8i) ₁ _(+4,8i) ₁ ₊₁₂ ⁽³⁾ i₂ i₁ 9 10 11 0-3 W_(8i) ₁ _(+12,8i) ₁_(+4,8i) ₁ ₊₁₂ ⁽³⁾ {tilde over (W)}_(8i) ₁ _(+4,8i) ₁ _(+12,8i) ₁ ₊₁₂⁽³⁾ {tilde over (W)}_(8i) ₁ _(+12,8i) ₁ _(+4,8i) ₁ ₊₄ ⁽³⁾ i₂ i₁ 12 13 140-3 W_(8i) ₁ _(+6,8i) ₁ _(+6,8i) ₁ ₊₁₄ ⁽³⁾ W_(8i) ₁ _(+14,8i) ₁ _(+6,8i)₁ ₊₁₄ ⁽³⁾ {tilde over (W)}_(8i) ₁ _(+6,8i) ₁ _(+14,8i) ₁ ₊₁₄ ⁽³⁾ i₂ i₁15 0-3 {tilde over (W)}_(8i) ₁ _(+14,8i) ₁ _(+6,8i) ₁ ₊₆ ⁽³⁾$\quad\begin{matrix}{{{{where}\mspace{14mu} W_{m,m^{\prime},m^{''}}^{(3)}} = {\frac{1}{\sqrt{24}}\begin{bmatrix}v_{m} & v_{m^{\prime}} & v_{m^{''}} \\v_{m} & {- v_{m^{\prime}}} & {- v_{m^{''}}}\end{bmatrix}}},} \\{{\overset{\sim}{W}}_{m,m^{\prime},m^{''}}^{(3)} = {\frac{1}{\sqrt{24}}\begin{bmatrix}v_{m} & v_{m^{\prime}} & v_{m^{''}} \\v_{m} & v_{m^{\prime}} & {- v_{m^{''}}}\end{bmatrix}}}\end{matrix}$

A codebook that is used by UE in order to report 4-layer CSI using theantenna ports 15 to 22 is given as in Table 8 below.

TABLE 8 i₂ i₁ 0 1 2 3 0-3 W_(8i) ₁ _(,8i) ₁ _(+8,0) ⁽⁴⁾ W_(8i) ₁ _(,8i)₁ _(+8,1) ⁽⁴⁾ W_(8i) ₁ _(+2,8i) ₁ _(+10,0) ⁽⁴⁾ W_(8i) ₁ _(+2,8i) ₁_(+10,1) ⁽⁴⁾ i₂ i₁ 4 5 6 7 0-3 W_(8i) ₁ _(+4,8i) ₁ _(+12,0) ⁽⁴⁾ W_(8i) ₁_(+4,8i) ₁ _(+12,1) ⁽⁴⁾ W_(8i) ₁ _(+6,8i) ₁ _(+14,0) ⁽⁴⁾ W_(8i) ₁_(+6,8i) ₁ _(+14,1) ⁽⁴⁾${{where}\mspace{14mu} W_{m,m^{\prime},n}^{(4)}} = {\frac{1}{\sqrt{32}}\begin{bmatrix}v_{m} & v_{m^{\prime}} & v_{m} & v_{m^{\prime}} \\{\phi_{n}v_{m}} & {\phi_{n}v_{m^{\prime}}} & {{- \phi_{n}}v_{m}} & {{- \phi_{n}}v_{m^{\prime}}}\end{bmatrix}}$

A codebook that is used by UE in order to report 5-layer CSI using theantenna ports 15 to 22 is given as in Table 9 below.

TABLE 9 i₂ i₁ 0 0-3$W_{i_{1}}^{(5)} = {\frac{1}{\sqrt{40}}\begin{bmatrix}v_{2i_{1}} & v_{2i_{1}} & v_{{2i_{1}} + 8} & v_{{2i_{1}} + 8} & v_{{2i_{1}} + 16} \\v_{2i_{1}} & {- v_{2i_{1}}} & v_{{2i_{1}} + 8} & {- v_{{2i_{1}} + 8}} & v_{{2i_{1}} + 16}\end{bmatrix}}$

A codebook that is used by UE in order to report 6-layer CSI using theantenna ports 15 to 22 is given as in Table 10 below.

TABLE 10 i₂ i₁ 0 0-3$W_{i_{1}}^{(6)} = {\frac{1}{\sqrt{48}}\begin{bmatrix}v_{2i_{1}} & v_{2i_{1}} & v_{{2i_{i}} + 8} & v_{{2i_{1}} + 8} & v_{{2i_{1}} + 16} & v_{{2i_{1}} + 16} \\v_{2i_{1}} & {- v_{2i_{1}}} & v_{{2i_{i}} + 8} & {- v_{{2i_{1}} + 8}} & v_{{2i_{1}} + 16} & {- v_{{2i_{1}} + 16}}\end{bmatrix}}$

A codebook that is used by UE in order to report 7-layer CSI using theantenna ports 15 to 22 is given as in Table 11 below.

TABLE 11 i₂  i₁  0 0-3$W_{i_{1}}^{(7)} = {\frac{1}{\sqrt{56}}\begin{bmatrix}v_{2i_{1}} & v_{2i_{1}} & v_{{2i_{i}} + 8} & v_{{2i_{1}} + 8} & v_{{2i_{1}} + 16} & v_{{2i_{1}} + 16} & v_{{2i_{1}} + 24} \\v_{2i_{1}} & {- v_{2i_{1}}} & v_{{2i_{i}} + 8} & {- v_{{2i_{1}} + 8}} & v_{{2i_{1}} + 16} & {- v_{{2i_{1}} + 16}} & v_{{2i_{1}} + 24}\end{bmatrix}}$

A codebook that is used by UE in order to report 8-layer CSI using theantenna ports 15 to 22 is given as in Table 12 below.

TABLE 12 i₂  i₁  0 0 $W_{i_{1}}^{(8)} = {\frac{1}{8}\begin{bmatrix}v_{2i_{1}} & v_{2i_{1}} & v_{{2i_{i}} + 8} & v_{{2i_{1}} + 8} & v_{{2i_{1}} + 16} & v_{{2i_{1}} + 16} & v_{{2i_{1}} + 24} & v_{{2i_{1}} + 24} \\v_{2i_{1}} & {- v_{2i_{1}}} & v_{{2i_{i}} + 8} & {- v_{{2i_{1}} + 8}} & v_{{2i_{1}} + 16} & {- v_{{2i_{1}} + 16}} & v_{{2i_{1}} + 24} & {- v_{{2i_{1}} + 24}}\end{bmatrix}}$

As described above with reference to Tables 4 to 12, the UE feeds thefirst channel information and the second channel information back to theBS. Here, the second channel information may be considered asinformation that particularly specifies the first channel information.In the above example, a case where the second channel informationindicates one precoding matrix of a set of precoding matrices indicatedby the first channel information has been illustrated as an example inwhich the first channel information and the second channel informationare combined and used, but the present invention is not limited thereto.A PMI for a downlink channel may be provided using another method ofcombining the first channel information and the second channelinformation.

Referring back to FIG. 10, the BS determines a precoding matrix to beused when performing MIMO transmission to the UE using the first channelinformation and the second channel information (S105). Next, the BStransmits a downlink signal to the UE by applying the determinedprecoding matrix (S106).

If a plurality of component carriers is aggregated and used in downlinkin a multi-carrier system, UE can generate and report the first channelinformation and the second channel information for each componentcarrier. Or, UE can generate and report the first channel informationand the second channel information for all of a plurality of componentcarriers not each component carrier. Or, UE can feed back channelinformation by combining the two types of methods. For example, UE canindividually generate the first channel information and the secondchannel information for a specific one of a plurality of componentcarriers, generate the first channel information and the second channelinformation for the remaining component carriers, and report them.

Meanwhile, an RI may be signalized separately from the first channelinformation and the second channel information or may be implicitlyinformed through the first channel information.

In the above method of transmitting CSI, if the UE does not transmit thefirst channel information, but transmits only the second channelinformation, there may be a problem in which the BS cannot determine adetailed PMI. The UE may transmit the CSI through a PUCCH or a PUSCH.Here, the UE may drop the transmission of specific CSI. For example, theUE may be configured so that it transmits the first channel informationthrough the PUCCH of a first subframe and transmits the second channelinformation through the PUCCH of a second subframe. If the number ofbits of ACK/NACK that have to be transmitted through the PUCCH of thefirst subframe is many, the UE may have to drop the transmission of thefirst channel information and to transmit only the ACK/NACK. In thiscase, the first channel information is not transmitted, and only thesecond channel information is transmitted to the BS. In this case, howthe BS will specify a precoding matrix preferred by the UE may beproblematic. A method of solving this problem is described below.

Method 1

If the transmission of first channel information (e.g., a first PMI) isdropped in a specific subframe, UE may generate and transmit secondchannel information (e.g., a second PMI) under the assumption of thefirst channel information that has most recently been transmitted beforethe specific subframe in a section until a subframe in which the firstchannel information has to be subsequently transmitted.

FIG. 11 illustrates the method 1.

Referring to FIG. 11, the first PMI may be configured so that the firstPMI is transmitted in a subframe n1, a subframe n5, and a subframe n9,and the second PMI may be configured so that the second PMI istransmitted in a subframe n2, a subframe n3, a subframe n4, a subframen6, a subframe n7, and a subframe n8. For example, the transmissionperiod of the first PMI may be 20 ms, and the transmission period of thesecond PMI may be 5 ms. Here, although the UE has transmitted the firstPMI in the subframe n1, the UE may drop the transmission of the firstPMI in the subframe n5. In this case, the UE generates the second PMI,transmitted in a section from the subframe n6 to the subframe n8, underthe assumption of the first PMI transmitted in the subframe n1.

If the first PMI is not received in a subframe in which the reception ofthe first PMI has been scheduled (e.g., the subframe n5), a BSdetermines a precoding matrix to be used for MIMO transmission using thefirst PMI that has most recently been received (i.e., the first PMIreceived in the subframe n1) and the second PMI at points of time atwhich it will has been received (i.e., the second PMI received in thesubframe n6 to the subframe n8).

In a multi-carrier system, if UE transmits the first channel informationand the second channel information for a plurality of DL CCs, when theUE drops the transmission of the first channel information for aspecific DL CC, the UE generate the second channel information for thespecific DL CC based on the first channel information that has mostrecently been transmitted for the specific DL CC and transmit thegenerated second channel information.

In the above example, a CQI may be generated and transmitted based on aPMI when the first PMI that has most recently been transmitted by the UEand the second PMI at points of time at which it is scheduled to betransmitted by the UE are combined. The CQI may include an SNR, an SNIR,or a preferred MCS.

Method 2

If UE drops the transmission of first channel information in a subframein which the first channel information has been scheduled to betransmitted, the UE may transmit second channel information which canindependently derive a PMI until a subframe in which the transmission ofthe first channel information has been subsequently scheduled.

FIG. 12 illustrates the method 2.

Referring to FIG. 12, the UE may be configured so that it transmits thesecond channel information in subframes n2 to n4 and n6 to n8 so that ittransmits the first channel information in subframes n1, n5, and n9. TheUE may drop the transmission of the first channel information in thesubframe n5 in which the transmission of the first channel informationhas been scheduled. Thus, the UE may generate the second channelinformation transmitted in the subframes n6 to n8 so that a PMI isindependently derived without being based on the first channelinformation and transmit the second channel information. For example, ifinformation of 8 bits is necessary to specify the PMI, the first channelinformation transmitted in the subframe n1 by the UE may provide thefirst PMI of 4 bits, and the second channel information transmitted inthe subframes n2 to n4 by the UE may provide the second PMI of 4 bits.

In contrast, the second channel information transmitted in the subframesn6 to n8 may provide the second PMI of 8 bits. Thus, a BS can identify aspecific PMI using the first PMI and the second PMI in the section ofthe subframes n1 to n4, but identify a specific PMI using only thesecond PMI in the section of the subframes n6 to n8. Here, a CQI isgenerated based on a PMI into which the second PMI that has mostrecently been transmitted has been incorporated and transmitted. The CQImay include information such as an SNR, an SINR, and an MCS.

Method 3

If UE drops the transmission of first channel information in a subframein which the first channel information has been scheduled to betransmitted, the UE may transmit the first channel information at thenext point of time at which second channel information will besubsequently transmitted. Or, the UE may transmit the first channelinformation that has been dropped at a specific point of time before asubframe in which the first channel information has been scheduled to besubsequently transmitted.

FIG. 13 illustrates the method 3.

Referring to FIG. 13, the UE may be configured so that it transmits thesecond channel information (e.g., a second PMI) in subframes n2 to n4and n6 to n8 so that the UE transmits the first channel information(e.g., a first PMI) in subframes n1, n5, and n9. Here, the UE may dropthe transmission of the first channel information in the subframe n5. Inthis case, the UE may transmit the first channel information that hasbeen dropped, in the subframe n6, that is, at a point of time at whichthe second channel information will be subsequently transmitted. Here,the second channel information that has been scheduled to be transmittedin the subframe n6 may be transmitted along with the first channelinformation or may be dropped. If both the first channel information andthe second channel information are transmitted in the subframe n6, a PMIfor the second channel information transmitted in the subframes n6 to n8may be determined based on the first channel information transmitted inthe subframe n6.

If the UE transmits an RI and the first channel information separatelyand generates the first channel information based on an RI, when the UEdrops the transmission of an RI at a specific point of time, the UE maygenerate the first channel information based on an RI that has mostrecently been transmitted before the specific point of time and transmitthe first channel information. If the UE transmits channel informationon a plurality of DL CCs, the UE may transmit the first channelinformation and the RI for each DL CC or DL CC group. Here, if thetransmission of an RI for a specific DL CC is dropped, the UE maygenerate the first channel information on the corresponding DL CC basedon an RI that has most recently been transmitted for the correspondingDL CC and transmit the first channel information.

FIG. 14 shows the construction of UE in accordance with an embodiment ofthe present invention.

A BS 100 includes a processor 110, memory 120, and a Radio Frequency(RF) unit 130. The processor 110 embodies the proposed functions,processes and/or methods. For example, the processor 110 transmitsconfiguration information through a higher layer signal, such as RRC,and transmits a reference signal, such as a CSI-RS. Furthermore, theprocessor 110 determines a precoding matrix using the first channelinformation and the second channel information that are fed back by UE,applies the precoding matrix to a downlink signal, and transmits thedownlink signal. The memory 120 is connected to the processor 110, andit stores various pieces of information for driving the processor 110.The RF unit 130 is connected to the processor 110, and transmits and/orreceives radio signals.

UE 200 includes a processor 210, memory 220, and an RF unit 230. Theprocessor 210 embodies the proposed functions, processes and/or methods.For example, the processor 210 receives configuration information from aBS and receives a reference signal. The processor 210 performs channelestimation for a channel with the BS using the reference signal andselects a precoding matrix within a codebook. The processor 210transmits information on the selected precoding matrix through the firstchannel information and the second channel information. The memory 220is connected to the processor 210, and it stores various pieces ofinformation for driving the processor 210. The RF unit 230 is connectedto the processor 210, and transmits and/or receives radio signals.

The processor 110, 210 may include Application-Specific IntegratedCircuits (ASICs), other chipsets, logic circuits, data processors and/orconverters for mutually converting baseband signals and radio signals.The memory 120, 220 may include Read-Only Memory (ROM), Random AccessMemory (RAM), flash memory, memory cards, storage media and/or otherstorage devices. The RF unit 130, 230 may include one or more antennasfor transmitting and/or receiving radio signals. When theabove-described embodiment is embodied in software, the above-describedscheme may be embodied into a module (process or function) that performsthe above function. The module may be stored in the memory 120, 220 andexecuted by the processor 110, 210. The memory 120, 220 may be placedinside or outside the processor 110, 210 and may be connected to theprocessor 110, 210 using a variety of well-known means.

Although the some embodiments of the present invention have beendescribed above, a person having ordinary skill in the art willappreciate that the present invention may be modified and changed invarious ways without departing from the technical spirit and scope ofthe present invention. Accordingly, the present invention is not limitedto the embodiments and the present invention may be said to include allembodiments within the scope of the claims below.

What is claimed is:
 1. A method for transmitting channel status information in a wireless communication system, the method performed by a user equipment (UE) and comprising: transmitting first channel information indicating a set of precoding matrices to a base station (BS) in subframes having a first period; and transmitting second channel information indicating one precoding matrix included in the set of precoding matrices to the BS in subframes having a second period, wherein the second channel information transmitted in a specific subframe is determined based on the first channel information that has most recently been transmitted before the specific subframe, wherein a channel quality indicator (CQI) transmitted in the specific subframe is calculated based on one precoding matrix specified by the first channel information that has most recently been transmitted before the specific subframe and by the second channel information determined based on the first channel information that has most recently been transmitted before the specific subframe, and wherein if the first channel information is dropped in a subframe included in the subframes having the first period, the second channel information transmitted after the subframe is determined based on first channel information that has most recently been transmitted before the subframe included in the subframes having the first period.
 2. The method of claim 1, wherein the first period is greater than the second period.
 3. The method of claim 1, wherein the first channel information and the second channel information are transmitted through a physical uplink control channel (PUCCH).
 4. A user equipment (UE), comprising: a radio frequency (RF) unit configured to transmit and receive radio signals; and a processor connected to the RF unit, wherein the processor is configured to: transmit first channel information indicating a set of precoding matrices to a base station (BS) in subframes having a first period, and transmit second channel information indicating one precoding matrix included in the set of precoding matrices to the BS in subframes having a second period, wherein the second channel information transmitted in a specific subframe is determined based on the first channel information that has most recently been transmitted before the specific subframe, wherein a channel quality indicator (CQI) transmitted in the specific subframe is calculated based on one precoding matrix specified by the first channel information that has most recently been transmitted before the specific subframe and by the second channel information determined based on the first channel information that has most recently been transmitted before the specific subframe, and wherein if the first channel information is dropped in a subframe included in the subframes having the first period, the second channel information transmitted after the subframe is determined based on first channel information that has most recently been transmitted before the subframe included in the subframes having the first period.
 5. The UE of claim 4, wherein the first period is greater than the second period.
 6. The UE of claim 4, wherein the first channel information and the second channel information are transmitted through a physical uplink control channel (PUCCH). 